Although some species of Legionella bacteria can be found in the soil, most species live in water that is stagnant and wherein such bacteria survive under a wide range of temperatures, typically 70 to 115 degrees F., according to some studies. The Centers for Disease Control and Prevention, USA, has reported that between 8,000 and 18,000 people are hospitalized with Legionnaires disease each year. It is of great public concern as its fatality rate during an outbreak ranges from 5% to 30% in those who contract the disease. Actively managing the risk of Legionella in water systems is more cost effective than responding to an outbreak. Outbreaks of Legionella pneumophila can stem from showers and potable water systems. As water from such sources aerosolized, individuals can inhale the Legionella containing droplets and the organism is aspirated into the lungs.
The formation and multiplication of such Legionella bacteria is not only promoted by the temperature in the customary hot water systems, but also by the fact that dead spaces are present in such water distribution systems in which deposits and sediment formation can arise, and typically in the bottom zone of water heater tanks. Deposits therein can represent a culture medium for bacteria proliferation.
Most electric water heaters for domestic use have its water tank constructed with a dome shaped bottom wall. Such dome-shaped bottom walls form a surrounding cavitated zone about the dome-shaped wall where sediments gather and where water is less agitated and most often stagnant. This cavitated zone becomes progressively narrow as the inner surface of the dome-shaped bottom wall merges towards the bottom end section of the tank circumferential side wall forming a narrow gap in which sediments accumulate and pile up to form a bed of sediments. This bed of sediments is spaced the furthest from the bottom heating element and thus water therein is less hot creating an ideal temperature location for bacterial proliferation. Should the bottom element fail, then the water temperature at the bottom of the tank will drop. In a study reported in 2011 and entitled “Sporadic Legionnaires disease: the role of domestic electric hot-water tanks”, by S. F. Dufresne, et al, and published by the Cambridge University Press 2011, water samplings were extracted from the drains at the bottom of several domestic water heaters and analysed. This study revealed a few positive samples at water temperature of 133 degrees Fahrenheit and that the bacteria was not present at temperatures of 135 degrees Fahrenheit and above. The World Health Organization recommends that hot water temperature be maintained above 131 degrees Fahrenheit. When hot water is not drawn from a water heater, the water inside the tank becomes stagnant and the water temperature stratifies with the cooler temperature being at the bottom region of the tank. Water within the cavitated zone below the bottom element of the tank can fall to about 85 to 105 degrees F. which is favourable to bacteria growth. Lowering the bottom element to place it close to the bottom wall of the tank has not proven to be a viable solution.
Reference is made to U.S. Pat. Nos. 4,940,024; 5,168,546 and 5,808,277 which disclose various methods and apparatus to prevent bacteria proliferation in electric water heaters. One method teaches adding a heating element in the form of a belt or patch on the outside of the tank against the bottom end of the outer sidewall of the tank to heat the water at the bottom end of the tank to a temperature preferably above 131 degrees F. Accordingly, this proposed solution provides an extra heating element in the form of a patch heater located in an area which is usually filled with insulating foam material and not practical to access should it fail and require replacement or repair. It is also costly and consumes more electricity. In U.S. Pat. No. 5,808,277 a third element is added into the tank to periodically raise the water temperature at the bottom of the tank beyond the pre-set consumption temperature, to a sanitizing temperature to destroy bacteria. This is also a costly proposition. U.S. Pat. No. 4,940,024 discloses a method of directing the cold water flow of all consumed drinking or domestically used water through the lower region of the tank wherein there is no stagnant water and wherein no deposits can be formed for bacteria growth. Accordingly, the lower region of the tank is continuously flushed with fresh water. This is a costly solution requiring a new tank design and cold water conduit network and therefore also not a viable solution.
The study reported in 2011 confirms that the bottom circumferential cavitated area between the bottom end section of the tank side wall and the dome-shaped bottom wall and this cavity gets progressively narrower down to its lower end and creating stagnant water niches for sediments to accumulate and form a culture bed and permissive environment for biofilm formation and proliferation of microorganisms, including free-living amoebas and Legionella. A solution to this problem thus becomes an urgent need.
In our recently filed U.S. patent application Ser. Nos. 15/731,020, filed Apr. 10, 2017 and 15/731,956, filed Sep. 1, 2017 there is described various heating means to heat the dome shaped bottom wall of the tank to prevent bacteria proliferation in the cavitated area. We also disclose the recirculation of hot water from the uppermost region of the tank where the water temperature is in the environment of 140 degrees F. to the bottom area of the tank for a predetermined time period to sanitize the bottom end of the tank during certain time periods and periodically. Because the prevention of the Legionella bacteria in such tanks is of utmost importance for people's health continuous research is ongoing in an attempt to find a solution to eradicate this public risk.